The present invention is generally related to control and operation of power converter devices, and, more particularly, to circuits for improving the operational performance of synchronous rectifiers used in DC/DC power converter devices.
DC/DC power converter devices are widely used in numerous applications, such as telecommunications and networking applications. A dc/dc converter is an electronics device that converts a raw dc (direct current) voltage input, usually with a certain variation range, to a dc voltage output that meets a set of specifications. With fast-growing technologies used in telecommunications equipment, the demands on the power density and conversion efficiency of dc/dc converters continue to increase.
FIG. 1 is a schematic diagram of a basic self-driven synchronous rectifier in one exemplary forward converter circuit topology. It is known that under certain conditions, the transformer secondary winding voltage levels could be either too low or too high to appropriately drive the synchronous rectifiers, SR1 and SR2. For example, since the input voltage (Vin) generally changes over a wide range, the driving voltage of the synchronous rectifiers can also change over a wide range. This undesirable voltage level variation for driving the synchronous rectifiers may result in detrimental power losses in the synchronous rectifiers and makes design optimization difficult to achieve. In addition, due to switching delays and other circuit parasitics, there may be an undesirable momentary xe2x80x9conxe2x80x9d condition that simultaneously occurs in both synchronous rectifiers SR1 and SR2. This could result in a relatively high spike of current xe2x80x9cshooting-throughxe2x80x9d in at least one of the synchronous rectifiers. Needless to say, such conditions could affect the efficiency, reliability and/or durability of the affected synchronous rectifier and associated components. It is also known to use a separate winding, instead of the main power winding of the power transformer, to drive both synchronous rectifiers SR1 and SR2. However, in such configuration, the charge and discharge of both synchronous rectifiers are inherently interdependent, making the simultaneous turn-on of rectifiers SR1 and SR2 very likely, and, thus, possibly, resulting in a xe2x80x9cshoot-throughxe2x80x9d condition.
Thus, it would be desirable to provide techniques and circuitry that, at relatively low-cost, improve the performance of each self-driven synchronous rectifier (SR) used in dc/dc power converters in order to advantageously reduce power losses and increase the overall efficiency of the power converter. It would be further desirable to provide techniques and circuitry that avoid or alleviate any xe2x80x9cshoot-throughxe2x80x9d conditions in the synchronous rectifiers.
Generally, the present invention fulfills the foregoing needs by providing in one aspect thereof a power converter device including a primary section and a secondary section electromagnetically coupled to one another through a transformer including respective primary and secondary transformer windings. In an exemplary forward converter, the primary section of the power converter includes a main power switch and a clamp switch respectively coupled to the primary transformer winding and configured to generally operate in mutually complementary on and off switching states, with some delays between them to avoid shoot-through and allow a switch""s drain-to source voltage to be discharged to a certain degree before it is turned on.
The secondary section of the power converter comprises a first synchronous rectifier coupled to the secondary transformer winding to pass a voltage induced at the secondary winding in response to an input voltage supplied to the primary transformer winding during an on-state of the main power switch. In one exemplary embodiment, a second synchronous rectifier may also be coupled to the secondary transformer winding to pass the voltage induced at the secondary winding when a center-tapped secondary winding is used, or to provide a path to free-wheeling current through an output inductor, if, for example, a single secondary winding is used during the on-state of the clamp switch. A first drive circuit is coupled to the gate terminal of the first synchronous rectifier to selectively activate and deactivate the first rectifier in correspondence with the respective on and off states of the main power switch based on a gate voltage supplied by the first drive circuit, with at least one circuit parameter being selected in the first drive circuit for maintaining the gate voltage within a predefined range over the variation in the level of the input voltage. Generally, the capacitance of a capacitor in the drive circuit is a preferred parameter to losslessly adjust the gate voltage. Moreover, using a capacitor in the drive circuit also allows the gate drive voltage to become proportional to voltage swings in the power transformer, and this reduces gate voltage sensitivity to input voltage variation since voltage swings in the power transformer usually vary over a narrower range than the input voltage. In the event a second synchronous rectifier is used, a second drive circuit is coupled to the gate terminal of the second synchronous rectifier to selectively activate and deactivate the second rectifier in correspondence with the respective on and off states of the clamp switch based on a gate voltage supplied by the second drive circuit, with at least one parameter being selected in the second drive circuit for maintaining the gate voltage within the predefined range regardless of variation in the level of the input voltage. The separation of the current paths for each respective gate drive circuit of the synchronous rectifiers allows substantial flexibility for optimizing their respective switching control timing.
In another aspect of the invention, a circuit for advancing turn-off of the synchronous rectifiers relative to turn-on of either the main power switch, or the clamp switch, or both, may be provided. This circuit is configured to avoid the possibility of a momentary high level of current passing therethrough or reduce its magnitude during a xe2x80x9cshoot-throughxe2x80x9d condition.